Biggest tip is to stop being offended and thinking to yourself "I cannot accept this sort of judgment." You yourself just said that you are not telling her about the opening of the relationship you share with your SO and you are "not sure I ever will." People cannot come to the conclusions that make you feel comfortable when you're intentionally keeping them from significant details, so learn to accept that your choice to withhold information to her changes the context within which she forms her moral beliefs about you.
You are right to say that you can't blame her. Now then, take that truth and start to work on your own hard-to-believe offense. I mean, look at your language carefully. "I am offended" is a passive voice construction: you are the direct object of someone else's verb, so in this instance, another offends you. This makes you the passive recipient or endpoint of another's actions. The change effected, they caused. This leaves you in your own language free to "be offended" since you're not the one responsible for generating the action, since you are the passive subject.
This is self-serving language. She is not offending. She is giving her deliberately uninformed opinion based on the information she does have. And who is deliberately not informing her? You are. You told us so. She is doing exactly what you want her to do, because you do not want to tell her the truth.
Even then, is it really "incredible" how offended you are? I think it's very common and believable for people to find offense when it enables them to distract themselves from their own responsibilities and commitments to ensuring others come to right moral decisions.
Offense is something we generate in ourselves, since we have a choice how to habituate ourselves to the responses we generate in others through our own actions and inactions. More so than having to choose to either "talk to her" or "back off," there's lots more options. You can stop falling in love. You can be honest and tell her why and how the relationship you share with your SO is open. You can learn the value in having a close friend of the opposite sex with whom there's no concern about the relationship becoming sexual. You can break up with your SO. You can bring your SO to the workplace and introduce them to one another.
There are so many more options, because right now, being a beginner, you only see the ups and downs to your days. You're not yet at that point where you see also the lefts, the insides, the beyonds, the withins, the surfaces, the fuzzies, the yellows. You don't yet see there are more options for how things change than up or down, talk or back off.
It is wise to back off, always. Inaction is much more difficult than action, and actionless action is the highest, but none of these are things you get without having first learned to back off, listen to your own self and your language, listen to the SO and the SO's language, listen to the Self whom you both are together, as the relationship you share with her. It's not just "my" relationship or "her" relationship if it takes the two of you in there to make it.
And that's part of what your epiphany shows: everyone has to benefit, and the only way to do that is to learn how to benefit the relationship comprising you and the SO. Do what's best for the Two-of-you, and not what's best for you within your relationships.
This epiphany will scale with however many take it upon themselves to think of themselves as within a relationship they share with others, an N-of-you.